1. Field of the Invention
The present invention is generally related to the field of drill bits used in drilling oil and gas wells, and, more particularly, to a fixed cutter drill bit with non-cutting erosion resistant inserts.
2. Description of the Related Art
Oil and gas wells are formed by a rotary drilling process. To that end, a drill bit is mounted on the end of a drill string which may be very long, e.g., several thousand feet. At the surface, a rotary drive mechanism turns the drill string and the attached drill bit at the bottom of the hole. In some cases, a downhole motor may provide the desired rotation to the drill bit. During drilling operations, a drilling fluid (so-called drilling mud) is pumped through the drill string and back up-hole by pumps located on the surface. The purpose of the drilling fluid is to, among other things, remove the earthen cuttings resulting from the drilling process.
There are two basic types of earth boring drill bits commonly used to form the boreholes in the earth for mineral exploration and recovery. The first utilizes one or more rolling cutters mounted on a bit body. There are typically several rows of cutting teeth on each cutter. When the bit body is rotated and weight is applied, the teeth on the cutters engage the earth causing the cutters to rotate. As the cutters rotate, the teeth are sequentially pushed into the earth effecting a drilling action. These bits are commonly known as rolling cutter drill bits or rock bits.
The second type of earth boring bit, and the subject of the present invention, utilizes cutting elements fixed on the blades of the bit body. These bits are also rotated, and when weight is applied, the cutting elements are pushed into, and dragged through the earth. This dragging action causes earth removal by shearing. These type of drill bits are generally known as fixed cutter drill bits.
There are different fixed cutter drill bit designs for different drilling applications. For example, a high bladed steel bit (often called a fishtail bit) may be suitable for rapidly drilling through very soft soils and formations, while a polycrystalline diamond compact (PDC) bit may be used to drill through harder rock formations. For very hard and tough rock formations, an infiltrated tungsten-carbide matrix bit body is employed with natural diamond cutting elements. These are typically called diamond or natural diamond drill bits.
As a general rule, drill bits that are able to drill rapidly through soft formations cannot penetrate the harder formations and, similarly, drill bits that are able to drill through harder formations are not aggressive enough to economically drill through softer formations. Thus, when drilling deep wells through many different types of rock and soil, drill bits may have to be changed many times in response to wear or in response to changing soil conditions.
Common to all types of earth drilling bits is a means to flush the drilled earth away from the cutting interface and transport it to the surface. For shallow boreholes, air is a suitable flushing fluid. However, for the deep boreholes commonly drilled for the exploration and production of oil and gas, the flushing fluid is typically a liquid. Because of its color and consistency, this liquid has come to be known as drilling mud or drilling fluid. Although the type of drilling fluid may vary, it typically contains abrasive elements, and it is usually pumped through nozzle orifices on the drill bit, typically at a rate of about 250 500 feet per second.
In rolling cutter drill bits, the primary role of drilling mud is to clean the bit and the bottom of the boreholes and transport the cuttings to the surface. In fixed cutter drill bits with PDC elements, however, the drilling mud has the added critical role of cooling the PDC diamonds. Clearly, diamond, and other suitable forms of superhard materials, are much harder than the earth formations being drilled, so theoretically these materials should not exhibit any wear. However, it is also apparent from examination of used drill bits that the superhard cutting elements do degrade. The degradation of the superhard cutting elements may be caused, at least in part, by the high temperatures generated at the cutting face from the friction of scraping rock. In order to minimize the degradation of the cutting faces, they must be cooled. For maximum cooling (and therefore minimum degradation), it is desirable to have the drilling fluid impinge directly on the cutting elements. However, PDC bits generally have exposed steel or infiltrated matrix surfaces adjacent to the diamond cutting elements, which can rapidly erode in the high velocity, abrasive laden stream of drilling fluid. There are numerous patents which show high velocity drilling fluids directed toward superhard cutting elements in steel bodied PDC drill bits, as shown, for instance, in U.S. Pat. Nos. 4,484,489; 4,907,662; 4,974,994; 4,883,136; 4,452,324; 4,303,136; as well as many others. Unfortunately, directing the flow of drilling fluid in this manner can cause severe erosion of the surfaces adjacent to the cutting elements.
For this reason, the nozzle orifices on PDC drill bits are typically oriented such that high velocity drilling fluid does not directly impinge the diamond cutting elements. Thus, although directing the drilling fluid at the diamond cutting elements on PDC bits would provide better cooling and longer life, commercial drill bits typically do not generally incorporate this feature because of erosion. Instead, the nozzle orifices typically direct the drilling fluid toward the formation at the bottom of the hole, and the splash is used to clean and cool the superhard cutting elements. As a consequence, typical PDC drill bits may not perform well where very high cutting element face friction is present, such as in hard rock drilling.
In addition, where soft, sticky formations are encountered, such as shales with high clay content, the hydraulic action of conventional PDC drill bits is sometimes inadequate to clean the cuttings away from the bit body and the cutting elements resulting in a phenomenon known as bit balling. Most drilling applications allow for between 100 hydraulic horsepower (HHP) and as much as 800 HHP at the bit. Optimizing the use of this significant source of energy to clean and cool the drill bit requires proper orifice size selection and proper placement of the nozzles, including optimum orientation.
In the past, there have been many different attempts to address the erosion problem described above. One common method to provide erosion resistance is to apply welded hardmetal in thick layers to the surface of the blades of a steel body drill bit. Unfortunately, welded hardmetal can crack as the blades of the PDC drill bit bend in response to the drilling loads. Once a crack starts, the impinging drilling fluid quickly erodes the exposed, soft underlying steel. Applying welded hardmetal is typically a hand-applied process and it is difficult to apply to the sides and bottom of the channels on the cutting face of PDC bits. Because it is a manual process, it is also subject to variation based on human and environmental factors. Once the welded hardmetal is applied, it is generally so thick and uneven that it affects the hydraulic flow of the flushing fluids. The swirls and flow eddies in the wake of these thick, rough layers can make the erosion problem even worse. Finally, the temperature caused by the welding process not only affects the heat treatment of the steel PDC bit bodies, as it can also cause the bodies to warp and even crack due to the thermal stresses. The temperature due to welding may also have a deleterious effect on the diamonds themselves if the DDC cutting elements are brazed prior to performing the welding process.
Another approach to erosion resistance is shown by Radtke in U.S. Pat. No. 4,396,077, herein incorporated by reference. Radtke describes a thick tungsten carbide coating applied to the cutting faces of PDC bit bodies with a high velocity plasma arc flame spray process. This process was considered an improvement over the conventional high velocity flame spray processes known at the time. Unfortunately, the problem with this and all other flame spray type coating processes is that the sprayed particle stream must impinge nearly perpendicular to the surface to be coated to make the coating adhere to the cutting face of the bit body. Although sprayed coatings can provide good erosion protection on some areas of the drill bit, the coating does not adhere well to the blade surfaces that are approximately parallel to the cutting face. PDC bits usually have channels formed in the cutting face for the high velocity flushing fluid. Since these channels usually have vertical walls, spray type coatings do not provide adequate erosion resistance in these areas of the drill bit. Also, a flame spray apparatus is generally a line-of-site type device and its powder coating discharge nozzle is normally located some distance away from the surface being coated. The irregular features on the cutting faces of most PDC bits cause “shadows” which block the spray path, preventing direct impingement by the spray. These limitations greatly reduce the effectiveness of the flame spray processes for producing wear and erosion resistant coatings on PDC bits.
Natural diamond drill bits (also called diamond drill bits) are very old in the drilling industry and provide an alternate way of addressing the wear and erosion problems of fixed cutter drill bits. This type of fixed cutter drill bit is made in an infiltration process. The resulting drill bit is generally referred to as a matrix bit body. In this process, natural diamonds or other very hard fixed cutting elements are inserted into cavities in a mold. Powders of highly wear and erosion resistant materials (typically including tungsten carbide) are then packed into the mold, and an infiltrant, typically a copper alloy, is placed in contact with the powders. The mold with the powders, cutting elements and infiltrant are all placed into a furnace and heated to the melting point of the infiltrant. The melted infiltrant fuses the diamonds and powders into a solid mass. This process produces a unitary body of infiltrated tungsten carbide and fixed cutting elements with improved wear and erosion resistance. By way of example, an early diamond drill bit design is disclosed in U.S. Pat. No. 2,371,489. It is also possible to form pockets in an infiltrated cutting face and later attach polycrystalline diamond cutters, as shown in U.S. Pat. No. 4,073,354, providing a somewhat more aggressive cutting structure than traditional diamond drill bits. Unfortunately, infiltrated or matrix bits are expensive to manufacture. Each bit must be cast in a mold in a very labor intensive process.
Infiltrated or matrix bit bodies are also weak in bending, so the blade height achievable with an infiltrated product is limited by the intrinsic strength of the material in bending. Therefore, these relatively shorter blades do not penetrate the earth as aggressively as the extended cutting faces of steel PDC drill bits. As a result, matrix drill bits do not provide the very high (and desirable) rates of penetration of PDC bits.
Finally, because the matrix drill bit products use a relatively soft copper based infiltrate to bind the tungsten carbide together, the infiltrated drill bit product can also be subject to erosion as the fluid stream attacks the copper binder, weakening the matrix and allowing tungsten carbide to be loosened from the body. The matrix design provides some erosion improvement over steel, but is still subject to all the limitations described above.
There are also numerous bit designs which are derivatives of either the infiltrated bit process or the coated steel process used in PDC bits. For example, in U.S. Pat. Nos. 4,554,130, 4,562,892 and 4,630,692, all herein incorporated by reference, a cladding process is disclosed for making a PDC type bit with a layer of wear and erosion resistant material. In these patents, a steel blank is coated with a thick layer of powders, the assembly is heated and then transferred to a press where the powders are fused to the steel surface under temperature and pressure with the aid of a ceramic or graphite pressure transfer medium. The layer must be thick, for it must contain a binder along with the wear resistant powder as it is compressed in the press. Although PDC type drill bits are shown and described in these patents, it is impractical to clad the vertical surfaces as shown. This is because the movement of the pressure transfer media tends to scrape the powders from the vertical steel surface as the press closes. Also, because the steel body itself is incompressible, the pressure transfer media will not be able to move in a manner which allows for an even pressure distribution. The end product of the above described cladding process has many of the same deficiencies as the flame spray coatings previously described, in that the vertical surfaces will not have adequate erosion protection.
The present invention is directed to an apparatus and methods for solving, or at least reducing the effects of, some or all of the aforementioned problems.